The field of the invention is an apparatus for alkylating benzene in a naphtha stream with dilute ethylene. The alkylated product may be used as motor fuel.
Dry gas is the common name for the off-gas stream from a fluid catalytic cracking unit that contains all the gases with boiling points of ethane and lower. The off-gas stream is compressed to remove as much of the C3 and C4 gases as possible. Sulfur is also largely absorbed from the off-gas stream in a scrubber that utilizes an amine absorbent. The remaining stream is known as the FCC dry gas. A typical dry gas stream contains 5 to 50 wt-% ethylene, 10 to 20 wt-% ethane, 5 to 20 wt-% hydrogen, 5 to 20 wt-% nitrogen, about 0.05 to about 5.0 wt-% of carbon monoxide, 0.1 to about 5.0 wt-% of carbon dioxide and less than 0.01 wt-% hydrogen sulfide and ammonia with the balance being methane.
Currently, the FCC dry gas stream is sent to a burner as fuel gas. An FCC unit that processes 7,949 kiloliters (50,000 barrels) per day will burn about 181,000 kg (200 tons) of dry gas containing, for example, about 36,000 kg (40 tons) of ethylene as fuel per day. Because a large price difference exists between fuel gas and motor fuel products or pure ethylene it would appear economically advantageous to attempt to recover this ethylene. However, the dry gas stream contains impurities that can poison catalysts and is so dilute that ethylene recovery is not economically justified by gas recovery systems.
There is need for utilization of dilute ethylene in refinery streams.
Catalytic reforming is a well-established hydrocarbon conversion process employed in the petroleum refining industry for improving the octane quality of hydrocarbon feedstocks, the primary product of reforming being motor gasoline. In catalytic reforming, a naphtha feedstock is admixed with a recycle stream comprising hydrogen and contacted with catalyst in a reaction zone at temperatures of around 493° to 510° C. (920° to 950° F.) and moderate pressure around 1379 to 3792 kPa (200 to 550 psig). The usual feedstock for catalytic reforming is a petroleum fraction known as naphtha and having an initial boiling point of about 46° C. (115° F.) and an end boiling point of about 204° C. (400° F.).
The catalytic reforming process is particularly applicable to the treatment of straight run gasoline comprised of relatively large concentrations of naphthenic and substantially straight chain paraffinic hydrocarbons, which are subject to aromatization through dehydrogenation and/or cyclization reactions. The catalyst “reforms” the molecular structures of the hydrocarbons contained in the raw naphtha by removing hydrogen and rearranging the structure of the molecules so as to improve the octane number of the naphtha. However, the increase in octane number also reduces the liquid volume of the naphtha as the specific gravity is increased. Because of the multiplicity of the compounds in the raw naphtha, the actual reactions which occur in catalytic reforming are numerous. However, some of the many resulting products are aryl or aromatic compounds, all of which exhibit high octane numbers. The aryl compounds produced depend upon the starting materials which in a refinery are controlled by the boiling range of the naphtha used and the crude oil source. The “reformed” product from a catalytic reforming process is commonly called reformate and is often separated into two fractions by conventional distillations, a light reformate having a boiling range of about 46° to 121° C. (115° to 250° F.) and a heavy reformate having a boiling range of about 121° to 204° C. (250° to 400° F.). The aryl compounds in each fraction are thus dependent upon their boiling points. The lower boiling or lighter aryl compounds, e.g., benzene, toluene and xylenes, are contained in the light reformate, and higher boiling aryl compounds are contained in the heavy reformate.
The concentration of benzene in gasoline is now being regulated by the American government. The Mobil Source Air Toxics regulation (MSAT II) requires that the average benzene level in gasoline produced by a refiner be lower than 0.62 vol-% with a maximum of 1.3 vol-% in gasoline produced at any one refinery. Benzene is commonly produced at levels higher than this by reforming processes and FCC processes. As reformate and the naphtha streams from the FCC unit are two of the largest sources of gasoline in a refinery, benzene reduction strategies have to be used.
Currently, benzene is commonly sent to a saturation unit to reduce benzene to cyclohexane. However, this process utilizes at least three moles of hydrogen for every mole of benzene converted and there is an octane loss associated with the conversion of benzene to cyclohexane. Methods for the reduction of benzene in gasoline without loss of octane or use of hydrogen are necessary.
The alkylation of concentrated benzene streams with concentrated ethylene streams is known. Alkylation typically involves the use of clean ethylene streams because alkylation catalysts are susceptible to feed impurities. Additionally, dilute ethylene is little used as an oligomerization feedstock because of its much lower reactivity relative to higher olefins. Benzene streams fed to alkylation reactors are also concentrated because of concern that heavier aromatics will preferentially alkylate, thereby requiring the use of a large excess of ethylene before reducing the benzene concentration and producing undesirable polyalkylated benzene.